The present invention relates to a test method and apparatus for a spark plug ceramic insulator.
Patent Publication 1 discloses a method for testing a spark plug ceramic insulator. This test method enables detection of the presence or absence of a defect in the spark plug ceramic insulator by the passage of an electric current between a first electrode placed in an inner hole of the ceramic insulator and a second electrode placed on an outer peripheral side of the ceramic insulator. More specifically, the test method includes the steps of generating a spark discharge through the application of a voltage between the first and second electrodes, allowing a path identification means to identify whether the spark discharge pass through an open end of the inner hole of the ceramic insulator and allowing a judgment means to judge the presence or absence of a defect in the ceramic insulator according to the identification result of the path identification means. The path identification means has a photoelectric conversion element and a light converging element arranged adjacent to the open end of the inner hole of the ceramic insulator and, when the spark discharge occurs between the first and second electrodes and passes through the open end of the inner hole of the ceramic insulator (i.e. in the occurrence of a so-called flashover phenomenon in which the electric discharge leaks out along a surface of the ceramic insulator), identifies the path of the spark discharge upon detection of light from the spark discharge.
[Patent Publication 1] Japanese Patent No. 2550790
In the presence of a defect such as a pin hole in the ceramic insulator, the spark discharge occurs due to the voltage difference between the first and second electrodes and passes through the defect rather than through the open end of the inner hole of the ceramic insulator. The path identification means identifies that the path of the spark discharge does not pass through the open end of the inner hole of the ceramic insulator. Then, the judgment means judges the presence of the defect in the ceramic insulator based on the identification result of the path identification means. In the absence of a defect in the ceramic insulator, by contrast, the spark discharge occurs due to the voltage difference between the first and second electrodes and passes through the open end of the inner hole of the ceramic insulator. The path identification means identifies that the path of the spark discharge passes through the open end of the inner hole of the ceramic insulator. The judgment means judges the absence of the defect in the ceramic insulator based on the identification result of the path identification means. In this way, the presence or absence of the defect in the ceramic insulator can be detected.
When the spark plug is manufactured through the above test method, the spark plug is judged as a conforming product with a proper required withstand voltage. Herein, the required withstand voltage of the spark plug is defined as the sum of an actual application voltage (actual operating voltage) of the spark plug during use in an internal combustion engine and a predetermined margin for accidental spark plug voltage application. For example, the required withstand voltage can be set to 30 V allowing for a margin on the actual operating voltage of 15 to 20 kV. The above voltage values are all direct-current voltage values. In the case of using an alternating-current power source as a test power source, the voltage value can be converted to a peak-to-peak voltage value.